Yep, that's the speedo plug. You remove the white insert and then insert the speedo when you want to measure boat speed through the water (instead of over the ground from your gps). It has not been glassed over, but added or repaired after the boat was built. I see some wires in the picture. Do any wires connect to a plug similar in size to the white plug, but have a paddle wheel at the bottom? You can change between the blank plug and the speedo with the boat in the water. Just use a towel or something to soak up the spill.

Definitely a plug for a Signet speed/knotmeter transducer. The red cap should turn out 1/4 turn or so and the plug lifts straight out. (it's O-ring sealed). Be sure to be ready to put it or a replacement back in in a hurry, you'll be amazed how much water tries to come up that little 1" hole!

Also...These types of openings, when under high pressure, have been shown to be an effective preventive backdoorifice when squatted over in a conscientiously applied program of regular (well, anal really) and professional care.

She is a Stiletto 27' The boat is nomex cored but where this plug is located is just solid glass/epoxy.

If it is a speedo plug, there is no outside hull evidence. Not even a subtle ridge or anything. Someone must have done a DAMN fine job of feathering out a thru hull patch. Also, there is an identical plug to this one in the other hull. If it is a speedo.... why two? It unscrews partially but runs out of threads and still doesn't lift out or loosen.

Bottom line question... I'm doing a major refit. Is this something I should remove or just let it be? I don't like the idea of thru hulls that may or may not still be able to leak.

It appears you are correct sir.... I googled the transducer and it is a dead ringer for what I got in my hull. That means there is a hidden thru hull there The hull looks very well patched on the outside (no evidence at all) but I should probably investigate further. BTW still curious why there are two???

Well, after some physical persuasion I was able to rip the center plug out of the thru hull. I'm VERY VERY glad I asked what the plugs were because only 2wimpy rotted o rings held back h2o from a 1" hole. They had likely been there a LONG time as the barrier coat was over them (hence not seeing any evidence of a thru hull for the outside) but they were NOT patched with FG. So now I need to post a new question. How does one remove an old thru hull???

Quote:

one was for probably for a depth transducer?

No, I already removed the fiberglassed in kitchen bowl filled with 30 year old mineral oil, disintegrating plastic transducer and a whole lot of funk.

Being that it's not a heavy displacement hull, I'd consider installing new 'O' rings in the caps. The surrounding area looks strong and those caps are probably factory installed and have worked just fine for years.

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